Engineering better organic solar cells

Organic solar cells based on conjugated (i.e., conducting) polymers have been attracting attention over the past several decades because they may provide a cost-effective route to wide use of solar energy for electrical power generation. These cells, or photovoltaics, have many advantages over traditional silicon-based solar cells. Specifically, conjugated polymer solar cells are easy and inexpensive to fabricate, lightweight, structurally flexible, and potentially applicable to large areas.

Despite considerable research effort, however, the power conversion efficiencies of such cells are still much lower than those of silicon-based devices. Currently, the best conjugated polymer photovoltaics achieve roughly one-quarter the efficiency of their traditional counterparts. Research toward understanding this class of organic semiconductors may lead to improvements in their power conversion efficiencies.

In the chemistry department at the University of Florida, research groups led by Kirk Schanze and John Reynolds collaborate closely to research new conjugated polymers and polyelectrolytes for photovoltaic applications. We also explore different solar cell structures using conjugated polyelectrolytes as functional materials.

In a typical solar cell of this type, the photoactive layer consists of two materials: an electron donor (the conjugated polymer) and an electron acceptor (usually a fullerene derivative). When a photon is absorbed by the polymer, an exciton, or a bound state of an electron and a hole (a complementary positive charge), is created. To ensure efficient exciton dissociation to generate free charge carriers, a small energy difference (∼0.4 eV) between the lowest unoccupied molecular orbit (LUMO) of the donor and that of the acceptor is required (see Figure 1). Thus, designing new conjugated polymers by tuning the bandgap energy involves two aspects: the energy level of the highest occupied molecular orbit (HOMO) and the energy difference between the HOMO and LUMO.

Figure 1. A difference in energy level between polymer donor and acceptor is essential for efficient exciton creation (1) and electron transfer (2). LUMO: Lowest unoccupied molecular orbit. HOMO: Highest occupied molecular orbit.

Power conversion efficiencies have been found to vary considerably depending on the polymers used,¹ emphasizing the importance of polymer structure for optimizing device performance. With this in mind, we proposed the concept of an ideal donor for photovoltaic devices and have been investigating the relationship between polymer structure and photovoltaic performance.^2–4 A series of polymers based on cyanovinylene and dioxythiophene were synthesized as representative examples, and the effects of their structures on photovoltaic response were found to be rather complicated. In a closely related area of research, we studied the platinum-incorporated conjugated polymer as a functional material.⁵ The relatively efficient response of the resulting photoactive layer was believed to involve photoinduced charge separation via the triplet excited state of the organometallic polymer in one of the first two reports on such a device.^5,6

Finally, we explored solar cell structures using conjugated polyelectrolytes as functional materials. Multilayer films were constructed using poly(phenylene ethynylene)-based anionic conjugated polyelectrolytes and a water-soluble cationic fullerene derivative.⁷ Under the standard AM1.5G test conditions, which mimic ambient sunlight at the Earth's surface, the cells exhibited power conversion efficiencies between 0.01 and 0.04% as a function of the thickness of the active layer. Additionally, the concept of spectral broadening has been demonstrated through the use of a dual-polymer system—poly(p-phenylene ethynylene) and polythiophene—to enhance the performance of a dye-sensitized nanocrystalline titanium dioxide solar cell.⁸

In summary, conjugated polymer photovoltaics provide a promising alternative to conventional silicon-based solar cells. However, further research on material and device engineering is needed to increase their efficiency. We will continue to study new, related organic semiconductors and different solar cell structures to better understand the relationship between materials and photovoltaic performance.